1. Field of the Invention
The present invention relates generally to electro-mechanical switch contacts and, more specifically, to an electrical contact assembly having helical coil contact structures.
2. Discussion of the Related Art
Industrial limit switches, such as global limit switches, safety interlocks, safety solenoid interlocks and cable pull limit switches are used in a variety of industrial applications. Limit switches contain switches that are manufactured to suit the particular application.
Typically, switches used in these products are designed and manufactured for a particular contact arrangement. Such switches generally contain two or more circuits having one or more normally closed (NC) contacts and/or one or more normally open (NO) contacts. Conventional arrangements typically include one normally open/one normally closed, also called a xe2x80x9csingle polexe2x80x9d arrangement, or two normally open/two normally closed, also called a xe2x80x9cdouble polexe2x80x9d arrangement. Conventional industrial limit switches often require additional normally closed contacts for added redundancy. Limit switch applications often require at least three positively driven, normally closed contacts along with one normally open monitor circuit.
U.S. Pat. No. 6,114,639, assigned to the assignee of the present invention, teaches a configurable switch having a configurable base and a configurable plunger, which together permit the adjustable arrangement of contacts into several electrical switching configurations. Within such a switch, one or more movable contacts are mounted to a plunger that is movable between a first position and a second position. In the first position or the second position, the movable contacts make contact and/or break contact with respective stationary contacts, depending upon the arrangements of the stationary contacts and the movable contacts. The movable contacts shown and typically used in such switch configurations are constructed from conductive material with each including two contact pads spaced at a distance from each other and connected by a contact bracket. The contact pads are each preferably composed of a fine silver disk or other suitable conductive material welded or otherwise attached to the movable contact. The contact bracket of each movable contact is mounted within a respective opening of the movable plunger and biased by a spring, thereby positioning the associated contact pads either apart from a respective stationary contact or engaged to the stationary contact, forming either a make or a break switching arrangement.
When such an electro-mechanical switch is used in low energy applications, typically those in the 24 volt, 2-10 milliamp range, other more-conductive materials or contact structures are used in order to enhance continuity and reliability. For example, gold plating may be deposited on a serrated contact, a bifurcated contact can be substituted for the contact pad, or a knife-edge shaped contact using a gold-inlay material substituted for the contact pads.
All of these material and structural substitutions, however, over the course of many switching operations, eventually fail in maintaining continuity, causing the low energy switch to fail. For example, gold-plating, deposited typically at a thickness of 0.0001 to 0.0002 inches, tends to wear off over the life of the contact. Serrated contacts attract contaminants from within the switch, which contaminants become trapped as the serrations degrade. Gold-inlay, which is very soft and ductile, also degrades over many mechanical operations of the contacts due to the xe2x80x9chammeringxe2x80x9d effect imparted by the mechanical closure of the contacts. Gold-plated bifurcated contacts, even though providing a somewhat more reliable contact, due primarily to its plural points of electrical contact, also eventually suffers from wear imparted by the hammering effect and contact bounce. Contact bounce in such switching contacts can lead to equipment malfunctions due to the fast response time of the industrial equipment being controlled.
Therefore, reliability of operation is important in such low-energy switching devices since these devices are extensively used in safety relay and safety control applications in industry. A marginally-operating or failed switch can have economic as well as catastrophic consequences to an industrial process if the devices fail to operate correctly.
It is, therefore, an object of the present invention to provide an improved electrical switch contact.
It is also an object of the present invention to provide an electrical switch contact assembly that increases the level of continuity in low-energy switch applications.
It is also a further object of the present invention to provide an improved switching contact assembly that minimizes contact bounce.
Therefore, there is provided in a first embodiment of the present invention, an electrical contact assembly comprising a bracket having at least a first and a second cavity located on opposite ends of the bracket and an opening extending through the bracket between the first cavity and said second cavity. The electrical contact assembly further includes an electrically conductive retention member mounted within the bracket opening having a first tubular portion extending outward from the first cavity, terminating in a first annular flange. A second tubular portion extends outward from the second cavity, also terminating in a second annular flange. The said first and said second flanges have a diameter greater than their respective and associated retention member first and second portions. A first coiled electrical contact is positioned about and compressively retained on the retention member first portion between the first annular flange and the bracket first cavity. A second coiled electrical contact is also positioned about and compressively retained on the retention member second portion, between the second annular flange and the bracket second cavity.
In a second embodiment of the present invention there is provided an electrical contact assembly comprising a bracket having at least a first and a second cavity located on opposite ends of the bracket and an opening extending through the bracket between the said first cavity and said second cavity. An electrically conductive retention member is mounted within the bracket opening and includes a first portion having first and second arms, oriented in a parallel and spaced relationship to the other, extending outward from the first cavity. A second portion, also having first and second arms, oriented in a parallel and spaced relationship to the other, extends outward from the second cavity. Each first and second arm of the first and second portions terminates in a wedge-shaped structure that extends perpendicularly from its associated arm, thereby forming a shoulder member having a width greater than the width of the first and second portions respectively. The electrical contact assembly further includes a first coiled electrical contact positioned about and compressively retained on the retention member first portion, between its associated shoulder member and the bracket first cavity and a second coiled electrical contact positioned about and compressively retained on the retention member second portion, between its associated shoulder member and the bracket second cavity.